Supplementary air cascade evaporation

ABSTRACT

A chemical recovery unit in which black liquor from a pulping process is burned including a direct contact evaporator for concentrating the black liquor prior to its introduction into a furnace. Preheated air is used as the heating medium in the direct contact evaporator. The moisture laden air leaving the evaporator is used as secondary combustion air in the furnace.

United States Patent Owens et al. 1 Nov. 28, 1972 [54] SUPPLEMENTARY AIRCASCADE [56] References Cited EVAPORATION v P 0w w S b UNITEDSTATESPATENTS 72 I t t t l 1 men ms Gm J Prohazka Si risb 3 2,516,992 8/1950Hochmuth ..23/262 of Com I 2,702,235 2/1955 Hochmuth ..23/262 3,047,3627/1962 Smith ..23/48 [73] Assignee: Combustion Engineering Inc.,Windsor, Conn. Primary Examiner-Norman Yudkoff Assistant Examiner- LSofer [22] Flled: 1970 Attorney-Carlton F. Bryant, Eldon l-l. Luther,Robert [21] APPL 83,102 L. Olson, Jon F. Carney, Richard H Berneike,Edward L. Kochey, Jr and Lawrence P. Kessler [52] US. Cl. ..159/4A,159/16, 159/11 B, [57] ABSTRACT 159/47 159,4 23/48 122/7 c A chemicalrecovery unit in which black liquor from a [51] Int. Cl. ..B0ld 1/16,BOld 1/14, BOld 1/22, pulping process is burned including a directContact Bold F22d evaporator for concentrating the black liquor prior to58 Field of Search ..159/47 WL, 4 A; 23/48, 262; its imwduflion into afurnace- Preheat air is used as 162/30; 122/7 C; 159/l6 A, 4 VM theheating medium in the direct contact evaporator.

The moisture laden air leaving the evaporator is used as secondarycombustion air in the furnace.

2 1 Figure l l 52 44 i 48 l 78 l l we/Auk) 4/19 26 66 70 ee 56601704,?!.40? L f 72 W'AK BLACK uquok co/vc. 1141/01? 65 70 X 50: ms

1 SUPPLEMENTARY AIR CASCADE EVAPORATION BACKGROUND OF THE INVENTION Inburning black liquor in chemical recovery furnaces, it is desirable tomaintain the solids content of 5 the black liquor within the range of65-70 percent in order to make it more easily burned and to bettercomplete the oxidation of malodorous sulphur compound to further theirconversion to recoverable salts. Many methods of concentrating the blackliquor prior to its introduction into the furnace have been used in thepast.

One method has been to use the combustion gases exiting from the furnaceto preheat combustion air. This preheated air is then passed through acontact evaporator to evaporate moisture from the black liquor, prior toits introduction into the furnace for supporting combustion. This systemachieves the high density black liquor desired, and also prevents theproduction of malodorous sulphur gases such as hydrogen sulphide,dimethyl sulphide, methyl mercaptan, etc., found in the exhaust gases ofsome furnaces. However, because of the large amount of moistureevaporated from the black liquor and carried into the furnace along withthe preheated air, there can be problems with combustion or flamestability. Since loss of combustion within the furnace can be hazardousand costly, loss of combustion should be avoided if at all possible.

SUMMARY OF THE INVENTION The chemical recovery unit of the inventionuses a direct contact evaporator for concentrating the black liquorprior to its introduction into a furnace. Air is preheated by theexhaust gases leaving the furnace. One portion of the preheated air isused for evaporating moisture from the black liquor in the directcontact evaporator, and is thereafter introduced into the furnace to beused as secondary combustion air. A second portion of the air flows fromthe air preheater directly to the furnace and is used as primarycombustion air. This moisture-free portion of the air insures flamestability, virtually eliminating the possibility of loss combustionwithin the furnace because of its relatively high temperature.

BRIEF DESCRIPTION OF THE DRAWING The FIGURE is a partial sectional sideview of a chemical recovery unit incorporating the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring to the drawing,denotes a recovery furnace having a hearth, or smelting zone 12. Air tosupport combustion within the furnace is introduced through primary airnozzles 15, and secondary air nozzles 14. These nozzles are supplied byducts 16 and 17, respectively. Black liquor is sprayed into the furnacethrough nozzles 18. The black liquor comes from a direct contact cascadeevaporator 20, which is supplied by means of inlet 22 with incomingblack liquor. The concentrated black liquor is transported to thefurnace through line 26.

After the black liquor is burned in the furnace, the hot combustiongases flow upwardly through the furnace, first passing over superheatersections 28 and 30,

and then flowing through the steam generating tubes 34. Water issupplied to these tubes by drum 38, and steam is removed from upper drum36 and flows to the superheaters. Steam is also generatred in tubes 40,which line the walls of the furnace. The combustion gases leave thefurnace through gas pass 42, thereafter dividing into ducts 44 and 46.These ducts carry the combustion gases to air preheaters 48 and 50,where the gases pass in heat exchange relationship with air supplied byfan 56 through ducts 52 and 54. The relatively cool combustion gasesleave the air preheaters by means of ducts 58 and 60, and are exhaustedto atmosphere through the stack 62.

Heated air leaves the air preheaters through ducts which merge into aplenum 64. From this plenum extends duct 17, which supplies primarycombustion air to the furnace. Also extending from plenum 64 is duct 66,which conveys hot air to the direct contact cascade evaporator 20. Afterthis air passes through the evaporator, evaporating moisture from theblack liquor in the process, it enters duct 16, which conveys it to thefurnace to be used as secondary combustion air. Bypass duct 68containing damper 70 is provided for supplying air to the secondarycombustion air nozzles 14 without the necessity of flowing through theevaporator 20, when desired.

Dampers 72 and 76 are located in air ducts l6 and 17 so that the amountof primary and secondary air can be varied as desired. Likewise, dampers78 and 80 are positioned in the combustion gas ducts 58 and 60 so thatthe amount of combustion gases flowing through the air preheaters 48 and50 can be controlled.

The operation of the system will now be described. Moisture isevaporated from the black liquor supplied to direct contact evaporator20. The concentrated liquor, with a solids content of 65-70 percent, issprayed into furnace 10 through nozzles 18. The liquor in falling to thehearth or smelting zone 12 is dried and partially combusted, with someof the volatiles being driven off. The combustion of the volatiles issupported by secondary combustion air supplied through nozzles 14. Themajor portion of the combustion takes place in hearth 12, with primaryair to support combustion being introduced into the smelt zone throughnozzles 15.

The combustion gases after passing through the various heat exchangersassociated with the furnace, divides, and passes through the two airpreheaters 48 and 50. A portion of the air from the preheaters passesthrough cascade evaporator 20, evaporating moisture from the blackliquor. The moisture laden air is introduced into the furnace throughnozzles 14, to act as secondary combustion air. A second portion of theair from the preheaters is conveyed directly to the furnace, andintroduced through nozzles 15 as primary combustion air. This dry,heated air stabilizes combustion of the black liquor, virtuallyeliminating the possibility of loss of flame. Bypass duct 68, and thedampers located in the various ducts, can be used to vary theproportions of the primary and secondary air.

The use of hot, dry air as primary air permits the reduction oftemperature of the black liquor to the sprays 18. This is an importantfactor because established data proves that low liquor spraytemperatures are necessary to maintain minimum levels of malodorouscompounds discharged from the furnace.

tion a furnace in which black liquor is burned, direct contactevaporation means for evaporating moisture from the black liquor priorto its introduction into the furnace, air preheater means in which airis indirectly heated by the waste combustion gases exhausted from thefurnace, first air conveying duct means connecting the air preheatermeans to the direct contact evaporation means for conveying hot airthereto to evaporate and concentrate the liquor, second duct means forconveying the moisture laden relatively hot air from the evaporatormeans to the furnace to be used as secondary air for supportingcombustiomand third air conveying duct means connecting the airpreheater means to the lower portion of the furnacefor conveying hot airdirectly thereto to be used as primary air for supporting combustion ofthe concentrated black liquor delivered to the furnace.

2. The combination set forth in claim 1, includingfourth duct meansbypassing the direct contact evaporation means, and damper means locatedin the second and fourth duct means.

1. A chemical recovery unit comprising in combination a furnace in which black liquor is burned, direct contact evaporation means for evaporating moisture from the black liquor prior to its introduction into the furnace, air preheater means in which air is indirectly heated by the waste combustion gases exhausted from the furnace, first air conveying duct means connecting the air preheater means to the direct contact evaporation means for conveying hot air thereto to evaporate and concentrate the liquor, second duct means for conveying the moisture laden relatively hot air from the evaporator means to the furnace to be used as secondary air for supporting combustion, and third air conveying duct means connecting the air preheater means to the lower portion of the furnace for conveying hot air directly thereto to be used as primary air for supporting combustion of the concentrated black liquor delivered to the furnace.
 2. The combination set forth in claim 1, including fourth duct means bypassing the direct contact evaporation means, and damper means located in the second and fourth duct means. 